Turbo molecular pumps are among the most widely used high speed vacuum pumps. The problems, task definition and its solution will be explained below using the example of turbo molecular pumps.
Turbo molecular pumps assure a pumping technology devoid of oil vapors due to their selective pumping behavior. If, however, a turbo molecular pump is stopped, oil vapors and other contaminants can travel from the forepressure side to the high vacuum side and cause considerable contamination there, which is perceived in a very disadvantageous fashion as initial contamination when the pump is switched on again. The contaminants, especially oil residues, entail a considerable lengthening of the pump-down time. This can be prevented if turbo molecular pumps are flooded after being switched off. Then the inside surfaces of the pump and of the recipient or chamber being pumped are acted upon by flooding gas, so that the pump-down time is essentially shortened when the pump is switched on again. Apart from that, the flooding causes a shortening of the coasting time of the rotor, a criteria which is of particular importance when pumping with magnetic bearings due to the absence of friction.
Turbo molecular pumps equipped with flooding arrangements are described in the DE-PS 18 09 902 and the publication "Vacuum Technology", 20th year of publication (1971), volume 7, page 201 and following.
It is evident from these publications that some flooding conditions can be optimally fixed, in order to attain as short a pump-down time as possible after the pump is again switched on.
An important problem, however, which deals with the flooding rate, has since been handled in an extremely unsatisfactory manner. The problem is--with what velocity the flooding gas is to be introduced into the pump in order to further improve the optimum flooding conditions and above all in order to avoid disadvantageous effects.
This problem exists especially in pumps with magnetic bearings. The criteria which must above all be dealt with, in this case, are the long coasting or run down-time, which is due to the absence of friction, and the limitation on extraneous forces acting upon the rotor.
The question of the flooding rate to begin with comes down to the cross-section of the valve opening. With a small cross-section, meaning when the pump is flooding too slowly, there results an excessive coasting time of the rotor, which in most applications can no longer be justified.
With a large cross-section, the forces which act due to the entering gas flow upon the rotor are so large that critical situations can arise in which, for instance, the rotor makes contact with the emergency bearings because of over-loading the axial bearing. With average cross-sections, the two disadvantageous effects cross over, so that no satisfactory solution can be achieved by varying the cross-section of the valve opening. Also, with the same constructional size of a pump, recipients or receivers being pumped having very different volumes can be connected, which additionally complicates the predetermination of optimum flooding conditions for a specific type of pump.
For pumps with conventional bearings, the conditions present themselves as follows. In pumps of small and average constructional size, a compromise between the requirements of sufficiently high flooding rate and forces of inconsiderable magnitude on the rotor can to some extent be realized, since ball bearings can carry high forces for short time periods. In large pumps, the flooding rate must be increased so that the flooding time period does not become too long. This, however, entails larger forces which means higher loads on the bearings and other serious disadvantages. For instance, the deformation of the rotor blades by the flooding forces increases with increase of the rotor diameter. Limits are set here already for reasons of strength of materials. In order to avoid contact with the spacer discs, the spacing between rotor and stator discs must be increased or other measures must be taken, which have negative effects upon the output characteristics of the pump.
Another possibility of varying the flooding rate consists in opening and closing the flood valve at timed fixedly preset intervals. This requires additional effort, which is not justified by the results. An optimum flooding process cannot be achieved even by this, since different parametric conditions, as for instance, recipient size, type and pressure of the flooding gas, and operation of the back-up pump, can not be taken into account. The same disadvantages as described above result depending upon the cycle times.